feat: create action

Hopefully it works?

README.md

@@ -0,0 +1,20 @@
+# Nix Flake outputs size
+
+Use `nix path-info` to query the size of flake outputs and produce a markdown report.
+
+You can post this report as a comment to the PR associated with the current branch and/or export the report as a markdown artifact.
+
+Requires `nix`, `jq`, `curl`, `sed` and `coreutils` to be in path.
+
+## Usage
+
+```yaml
+- name: Generate size report
+  uses: https://git.salame.cl/jalil/nix-flake-outputs-size@main
+  with: # Default values
+    comment-on-pr: 'true'
+    generate-artifact: 'false'
+    artifact-name: 'size-report.md'
+```
+
+For more details see the [action.yaml](./action.yml) file.